1. Field of the Invention
The present invention relates to a printer which is capable of expressing halftones by halftone spots formed by a multi-value dither method, and an image processing device for performing the processing for the halftone expression.
2. Description of the Related Art
There is proposed a printer, such as an electrophotographic machine, in which halftone spots are formed by the multi-value dither method and halftones are expressed by using the areas of halftone spots. In a method of forming halftone spots by the multi-value dither method, a cell is formed by using a plurality of pixels adjacently arranged. An area of a dot formed within each pixel is determined in accordance with a gradation value of image data of the pixel. As a gradation value of image data increases, a dot within a pixel located at the center of a cell formed with a plurality of pixels is first grown, and then the dots in the pixels located around the center pixel are grown. In this way, the areas of the halftone spots are gradually increased to thereby express halftones. By expressing the dots formed within the pixels in multi-gradation levels, the number of gradation levels is increased even if the number of dots per unit area is small. Further, the halftone spot size is reduced and the number of screen lines is increased, whereby an image plane resolution is increased.
A color laser printer, which utilizes a laser beam, increases its market share in later years. In the color laser printer, a dot area in the pixel corresponds to a pulse width of a drive pulse for a laser beam to be irradiated onto a developing drum. Image data of each pixel is converted into drive pulse width data as image reproduction information by a halftone processing section in a controller. And, the drive pulse width data is converted into a drive pulse signal by a pulse width modulator, and the converted one is output to a print engine.
FIG. 1 is a block diagram showing an overall configuration of a conventional printer system. Character data, graphic data, bit map data and the like are generated by an application program 12 in a host computer 10. Upon receipt of those pieces of data, a printer driver 14 generates image data S14 representative of a gradation level of each pixel. The image data generated is contained in a print job and output to a printer 20.
The printer 20 includes a controller 22 and a print engine 30. The controller 22 interprets a print jot; it extracts the image data S14 and print commands; it stores the image data S14 into an image memory 23; it performs a color conversion of the image data in synchronism with the operation of the print engine 30; it carries out a half one process; and it outputs a drive pulse signal S29 for a laser diode 32 in the print engine. Accordingly, the controller 22 includes a color conversion section 24, a color conversion table 25 for color conversion, a halftone processing section 26, and a halftone table 28 which is referred to in executing the halftone process by the halftone processing section.
In the color conversion section 24, RGB image data is converted into CMYK image data S24. In the halftone processing section 26, 8-bit CMYK image data of each pixel is converted into pulse width data S26 of a drive pulse signal for driving the laser diode, which is used for forming a virtual dot in each of pixels forming a halftone spot capable of expressing the halftone. A pulse width modulation element 29 generates a drive pulse signal S29 by using the drive pulse width data S26. The halftone processing section 26 converts the 8-bit CMYK image data into the 8-bit drive pulse width data S26. Accordingly, the number of gradation levels. In this halftone process, the number of gradation levels (256 gradation levels) of each pixel is not changed.
In the print engine 30, the laser diode 32 generates a laser beam in accordance with a drive pulse signal, and a surface of the color conversion section 24 is scanned with the laser beam. Toner attaches to virtual dots in the pixels irradiated with the laser beam, so that dots are formed within the pixels. Those dots are printed on a printing medium, such as a printing sheet of paper. The developed dots form halftone spots, and halftones are expressed by using the areas of halftone spots.
In the present specification, the term “pixel” is an area to which image data, such as gradation value, is assigned, and the image data halftone processed is also generated corresponding to the pixel. In the print engine, a laser beam is irradiated onto the pixel on the photosensitive drum. The area irradiated with the laser beam is referred to as a “virtual dot”. Accordingly, the virtual dot is an area defined by a drive pulse signal for the laser beam. On the photosensitive drum, toner attaches to the virtual dots irradiated with the laser beam to thereby form “dots” formed by the toner. In other words, the dot is an area defined by the toner attached to within the pixel. The toner dots are transferred onto a printing medium, and a “halftone spot” is formed by dots formed by a plurality of pixels, whereby a halftone is expressed. A set of pixels, which form a halftone spot, will be referred to as a “halftone spot cell”.
In the case of the color printer, the controller 22 sequentially outputs drive pulse signals S29 for the image data of four colors, C, M, Y and K. The print engine 30 repeats the operations of forming color images of four colors, four times. Toners of four colors which have been transferred to an intermediate transfer medium (not shown) are transferred onto a printing sheet.
FIG. 2 is a diagram showing a configuration of a color printer of the tandem type. The color printer performs a multi-color printing process. Accordingly, it has a problem that the printing time is long. To solve the problem, a color printer of the tandem type has been proposed. As shown in FIG. 2, the color printer includes, to process images of four colors, four halftone processing sections 26, four pulse width modulation elements 29, four laser diodes 32, and four photosensitive drums 24. The controller concurrently generates CMYK drive pulse signals, color images of four colors are concurrently formed on the four photosensitive drums 24, and then are transferred onto intermediate transfer media 26. Thus, a time taken for the printing by the color printer of this type is shorter than that by the color printer of the type in which the color images are serially formed.
The color printer of the tandem type needs four sets of hardwares. In particular, use of the four pulse width modulation elements 29 is not preferable since it will increase the cost to manufacture. Further, the printer needs the four halftone processing sections 26, and hence, four halftone tables 28. The halftone table 28 is usually formed of a high speed semiconductor memory, such as a static RAM. This fact also increases the cost to manufacture.